MicroRNAs (miRNAs) were first identified only a few years ago, but already it is clear that they play an important role in regulating gene activity. These 20-22 nucleotide noncoding RNAs have the ability to hybridize via base-pairing with specific target mRNAs and downregulate the expression of these transcripts, by mediating either RNA cleavage or translational repression.
Recent studies have indicated that miRNAs have important functions during development. In plants, they have been shown to control a variety of developmental processes including flowering time, leaf morphology, organ polarity, floral morphology, and root development (reviewed by Mallory and Vaucheret (2006) Nat Genet 38: S31-36). Given the established regulatory role of miRNAs, it is likely that they are also involved in the control of some of the major crop traits such drought tolerance and disease resistance.
miRNAs are transcribed by RNA polymerase II as polyadenylated and capped messages known as pri-miRNAs. These pri-miRNAs are processed into smaller transcripts known as pre-miRNAs and these precursors have the ability to form stable hairpin structures (reviewed by Bartel (2004) Cell 116: 281-297; Jones-Rhoades M W, Bartel D P, Bartel B. MicroRNAS and their regulatory roles in plants. Annu Rev Plant Biol. 2006;57:19-53.) While pri-miRNAs are processed to pre-miRNAs by Drosha in the nucleus and Dicer cleaves pre-miRNAs in the cytoplasm in metazoans, miRNA maturation in plants differs from the pathway in animals because plants lack a Drosha homolog. Instead, the RNase III enzyme DICER-LIKE 1 (DCL1), which is homologous to animal Dicer, may possess Drosha function in addition to its known function in hairpin processing (Kurihara and Watanabe (2004) Proc Natl Acad Sci 101: 12753-12758).
Artificial microRNAs (amiRNAs) have recently been described in Arabidopsis targeting viral mRNA sequences (Niu et al. (2006) Nature Biotechnology 24:1420-1428) or endogenous genes (Schwab et al. (2006) Plant Cell 18:1121-1133). The amiRNA construct can be expressed under different promoters in order to change the spatial pattern of silencing (Schwab et al. (2006) Plant Cell 18:1121-1133). Artificial miRNAs replace the microRNA and its complementary star sequence in a precursor miRNA and substitute sequences that target an mRNA to be silenced. Silencing by endogenous miRNAs can be found in a variety of spatial, temporal, and developmental expression patterns (Parizotto et al. (2007) Genes Dev 18:2237-2242; Alvarez et al. (2006) Plant Cell 18:1134-51). Artificial miRNA can be constructed to both capture and extend the diversity and specificity in the patterns of silencing. To date there have been no reports of using amiRNAs in crop plants.
WO 2004/009779 published Jan. 29, 2004 describes compositions and methods for modulating gene expression in plants.
Applicant's Assignee's US Patent Application Publication 2005/0138689 published on Jun. 23, 2005 describes miRNas and their use in silencing a target sequence.